In the "old days," when programmers worked on printing terminals,
editing was done one line at a time. Editors that let you move
a cursor around the screen to select text to edit weren't invented,
because there weren't any screens to look at text on!

In these days of even more advanced WYSIWYG (What You See Is What
You Get) word processors an editing programs, it's easy for novices
to think of line editors as a bizarre relic. Perhaps they are - but
if so, they are a relic of extraordinary power.

You see, line editors lend themselves to
scripting
-the ability
to write what in effect are editing programs that can be applied over
and over to different files.

When we talk about "batch editing" or scripts, here are some of the
programs you might use:

ed
is the original UNIX line editor.

ex
supports a superset of
ed
commands; it is widely used from
within
vi
, which is the
ex
"visual" or "screen" mode.

sed
is an editor that can
only
be run with scripts [or by
entering a few short commands as command-line arguments -
JP
]; while
it has many similar commands, it has some
important differences (
34.1
)
from
ed
and
ex
.

patch
(
33.9
)
is a specialized editor designed to apply editing scripts created
with
diff
(
28.1
)
.
You can do this with
ed
or
ex
as well, but
patch
is
especially clever at it.

Of course, editing is a continuum, and beyond
sed
, you can
think of
awk
(
33.11
)
and
perl
(
37.1
)
as even more powerful editing programs.